It is common for anisotropy to exist across the surface of structural materials. For example, the stress levels in a steel plate which has undergone directional work, such as rolling, will typically vary substantially when measured in different directions across the surface of the plate. Such anisotropy in the planar stress of a material is commonly referred to as biaxial stress and may be represented in a well known manner by two orthogonal stress vectors .sigma..sub.x and .sigma..sub.y. Likewise, when a specimen has been subjected to directional loading, there will be anisotropy in the magnetic properties. In addition, the microstructure of the plate material may also vary in a directional manner across the surface.
Prior to the present invention, magnetic inspection probes have typically consisted of a substantially "C" shaped core wrapped with a power coil thereby generating a magnetic field in only one direction in the test specimen. An inspection apparatus utilizing one such uni-directional probe is illustrated in K. Kwun and G. L. Burkhardt, "Effects of Grain Size, Hardness, and Stress on the Magnetic Hysteresis Loops of Ferromagnetic Steels," J. Appl. Phys., Vol. 61, No. 4 (1987). As will be appreciated, in order to measure directional variations in the magnetic properties of a material, such a uni-directional probe must be reoriented with respect to the surface of the sample before each measurement is taken.
The magnetic inspection probe of the present invention is particularly useful in the evaluation of biaxial stress. However, anisotropy with respect to other physical characteristics--for example, texture or preferred grain orientation--may also be evaluated. Methods for measuring the physical characteristics of a material by means of magnetic evaluation have recently become known in the art. For example, U.S. Pat. No. 5,008,621 to Jiles discloses a system and method for use in the evaluation of the bulk magnetic properties of a material to obtain meaningful information regarding intrinsic physical characteristics.
The system of the '621 patent subjects a sample of a material to a magnetic field and takes multiple measurements of the magnetic field and magnetic flux of the specimen as the magnetic field is cycled in a controlled manner. As will be recognized, such a controlled cycling permits the collection of data sufficient to generate a magnetic hysteresis curve. As is well known, the magnetic hysteresis curve is a plot of flux density B in a material versus a varying applied magnetic field intensity H.
From the features of the hysteresis curve, an evaluation of the physical properties of a material can be made. For example, it is known that coercivity can be used to detect plastic deformation and hardness, that maximum differential permeability can be used to measure stress, that a combination of remanence and coercivity can be used to detect impending fatigue failure and that hysteresis loss can be used to detect changes in grain boundary segregation arising from temper embrittlement.
The system and method of the '621 patent to Jiles are useful in evaluating the overall bulk properties of a material. However, because the measurements taken with regard to specimen flux and field intensity have no specific directional component, directional anisotropy in features such as stress or microstructure is not identified.
U.S. Pat. No. 5,012,189 also to Jiles discloses a specific method for deriving information regarding stress from a ferromagnetic material. Specifically, the '189 patent discloses the ability to derive a meaningful estimate of the actual and residual stress occurring in a material based on the hysteresis and anhysteretic magnetization curves at the origin as compared to such curves in an unstressed sample. While the method for stress evaluation disclosed in the '189 patent is of great use, and is incorporated herein by reference, no means is disclosed therein for measuring any directional variation in stress or other intrinsic properties.
U.S. Pat. No. 5,059,903 to Otaka et al. discloses a system and method for evaluation of the embrittlement of a material through comparison of the magnitude of magnetization characteristics measured in a sample specimen to the magnetic characteristics of a virgin specimen. By making these analyses on a periodic basis, it is possible to determine degradation rates as well as to identify areas in which degradation is most severe.
While the '903 patent to Otaka et al. is useful in determining the location of degradation, no means is provided which would be capable of measuring in-plane anisotropy without taking multiple measurements in different directions.